1. Field of the Invention
The present invention relates to an image processing method for processing an image in digital manner and an apparatus therefor, and more particularly to an image processing method for obtaining a high-quality image from image density data and an apparatus therefor.
2. Related Background Art
So-called digital copying machines, in which an image is sampled by a CCD sensor or the like and the obtained digital data are supplied to a digital printer such as a laser beam printer for reproducing the image, are replacing conventional analog copying machines, owing to the progress in the digital devices.
The reproduction of intermediate tone in such digital copying machines is generally achieved by a dither method or a density pattern method. However, such methods have been associated with the following drawbacks:
(1) if the original image is a screen-tone image such as a printed image, the copied image may show stripe patterns which do not exist in the original image; and
(2) if the original image contains line-tone images or characters, the image quality may be deteriorated as the edges are broken by the dither method.
The phenomenon (1) is called Moire and is induced by:
(A) a beat phenomenon between the screen-tone original image and the input sampling; or
(B) a beat phenomenon between the screen-tone original image and the dither threshold matrix.
The phenomenon (b) becomes particularly evident when the dither threshold values are arranged in a dot concentrated pattern. In such case the reproduced image has a pseudo-screentone structure, which generates a beat phenomenon with the screentone structure of the input image, thus creating Moire patterns.
As another binary digitizing method there is already known the error dispersion method, in which the difference or error in density between the original image and the reproduced image is calculated for each pixel, and said error is dispersed in the surrounding pixels with a predetermined weighting. This method was published by R. W. Floyd and L. Steinberg in "An Adaptive Algorithm for Spatial Grey Scale", SID 75 DIGEST p. 36-37 . In comparison with the aforementioned dither method, this method lacks periodicity in the processing and does not, therefore, generate Moire patterns from a screentone original image. It is, however, associated with the drawbacks of showing particular stripe patterns in the reproduced image or providing granular noise particularly in the highlight and shadow areas of the image.
In order to eliminate these drawbacks, in U.S. Pat. No. 4,876,610, the assignee of the present application has proposed a method of varying the error dispersing area according to the characteristic of the original image, and U.S. Pat. No. 4,878,125 of the assignee of the present invention has proposed a method of varying the ratio of distribution of the error in relation to the amount and direction of edges in the image.